1. Field of the Invention
The present invention concerns the fabrication of opto-electronic components such as vertical cavity surface emitting laser (VCSEL) type semiconductor lasers. Other components to which this invention relates are microcavity light-emitting diodes and electrically tunable optical filters. This invention is more particularly concerned with the fabrication of such components when the light emitted or filtered must have a wavelength of about 1.3 .mu.m or 1.5 .mu.m so that it is in one of the spectral windows routinely used in fiber optic communication systems. The operation of the component at one of these wavelengths is obtained with the aid of an active structure that is the seat of the specific opto-electronic interaction of the component and which is formed within a monocrystalline indium phosphide (InP) wafer. This material has a cubic crystal lattice.
However, some parts of a laser or other component of this kind are advantageously made of gallium arsenide and/or other materials that also have a cubic crystal lattice but lattice matched to gallium arsenide, the lattice constant of which is different from that of indium phosphide. This applies in particular to a Bragg mirror constituting one end of the optical cavity of a laser or a microcavity light-emitting diode. It also applies to an electrical confinement layer for confining or equalizing the density of the excitation current that injects charge carriers into the optically active layer included in the cavity. A problem of crystal compatibility then arises because good crystal quality appears to be essential to correct operation of the laser, both with regard to a semiconductor wafer that is to constitute the active structure of a laser and with regard to other components such as a semiconductor Bragg mirror. The difficult problem is that of achieving a suitable connection of the crystal lattice of the gallium arsenide to that of the indium phosphide, suitable in the sense that it must be compatible with the required correct operation.
Various processes have been proposed or used to manufacture a surface emitting laser emitting at the wavelengths referred to above but circumventing or solving the problem referred to above.
2. Description of the Prior Art
A first prior art process avoids this problem, at least insofar as the Bragg mirror of the laser is concerned, by constructing this mirror from alternate layers of indium phosphide and a GaInAsP quaternary material lattice matched to the indium phosphide. The small difference between the refractive indices of the two materials then necessitates the formation of a large number of pairs of layers, given the high reflectance that is necessary. This number is in excess of 40 and leads to a narrow optical band of the mirror and a high electrical resistance. The latter is problematical if the excitation current is injected through the mirror. The quaternary material also introduces a resistance impeding the evacuation of heat. This first prior art process is described in "High reflectivity semiconductor mirrors for 1.3 .mu.m emitting lasers", P. Salet, C. Starck, A. Pinquier, Cleo 96.
The same problem is avoided by lattice matching in a second prior art process in which the materials of the Bragg mirror are AlInAs and GaAlInAs. The drawbacks of this second prior art process are similar to those of the first.
The same problem is also avoided by lattice matching in a third prior art process in which the materials of the Bragg mirror are AlAsSb and AlGaAsSb. Unfortunately, these materials tend to break down into two separate phases. This third prior art process is described in: "AlAsSb: AlGaAsSb Bragg stacks for 1.55 .mu.m wavelength grown by MBE", J. C. Harmand, F. Jeannes, G. Le Roux and M. Juhel Elec. Lett. Vol 31, 1995, 1669.
A fourth prior art process solves the problem in question, that is to say the problem that the crystal lattice of the Bragg mirror is lattice matched to gallium arsenide and is joined to an indium phosphide wafer. The materials of this mirror are aluminum arsenide AlAs and gallium arsenide. They produce a high reflectance and low thermal and electrical resistances. The mirror is formed by a process of epitaxial deposition on a second wafer of gallium arsenide. This second wafer is welded to the first by pressing them together at 600.degree. C. in an atmosphere of hydrogen, the combination of the two wafers when bonded in this way being referred to as a "composite wafer".
This fourth prior art process is costly because the dimensions of the wafers that can be welded in this way are limited. Lasers are conventionally fabricated economically by making large wafers which can subsequently be cut up so that each of the separated parts constitutes a laser or one or more laser matrices. Because of their limited dimensions, the composite wafers formed by this fourth prior art method cannot each constitute a sufficiently large number of lasers for the method to be economic. Moreover, the threshold voltage of the fabricated lasers is relatively high. This fourth process is known internationally as the "wafer fusion" process and in France as "fusion bonding". It is described in an article by D. I. Babic, K. Treubel, R. P. Mirin, N. M. Margalit, J. E. Bowers, E. L. Hu, D. E. Mars, L. Yang and K. Carey, Photon. Techn. lett. vol 7, 1225, 1995.
One aim of the present invention is to enable the manufacture at low cost of surface emitting lasers or microcavity light-emitting diodes emitting at the wavelengths associated with indium phosphide but incorporating a Bragg mirror or other component having inherent qualities associated with gallium arsenide and with lattice matched materials in a way that prevents bonding two crystal lattices with different lattice constants compromising the performance of the lasers or the diodes. A more general aim of the present invention is to provide a simple and effective way of associating these two types of materials for the manufacture of a component of this kind.